Resignation before a disciplinary

Hi, Can you advise on this please? We have an employee who is due to attend a disciplinary tomorrow due to a breach of data and capability performance issues. They resigned today, giving 4 weeks notice. I intend to continue with the disciplinary however my question is. Giving that the intention was to dismiss the employee, giving one weeks notice as per the employers contractual terms, can this still be done despite the resignation? Does the resignation affect the decision to dismiss an employee? The manager doesn’t want to keep the employee for another four weeks due to the reoccurring mistakes. However we can’t afford to pay the employee in lieu either. Employee has been employed for 9 months. Hope you can help with this. Laura